During the last decade, the rapidly progressing area of nanotechnology has shown that dimensionality plays a crucial role in determining the properties of a material. J. Hu, T. W. Odom, and C. M. Lieber, Acc. Chem. Res., 1999, 32, 435. The large spectrum of newly synthesized structures with one or two dimensions in the nanometer scale opens large fields for synthesis of new materials for electronic and computer technology. Nanowires and nanotubes appear to be quite attractive in this respect due to their unique optical, magnetic and electrical properties. M. S. Dresselhaus, G. Dresselhaus, P. C. Eklund, Science of Fullerenes and Carbon Nanotubes, Academic Press, San Diego 1996; C. M. Lieber, Solid State Commun. 1998, 107, 607; N. Agrait, A. L. Yeyati, J. M. Ruitenbeek, Phys. Rep. 2003, 377, 81. Micro- and nanostructured materials formed by assembly of colloidal particles could find a wide range of applications ranging from photonics and electronics to catalysis, bioprocessing, sensors, and energy storage. F. Caruso, Ed., Colloids and Colloid Assemblies. Synthesis, Modification, Organization, and Utilization of Colloid Particles, Wiley-VCH, Weinheim 2003, Ch. 8-15. The functionality of these materials largely depends on the size, shape and physical properties of the particles from which they are assembled. The use of anisotropic particles is of particular interest, as it allows the creation of materials of advanced microstructure or anisotropic properties.
Dispersions of polymer cylinders with colloidal sizes have unique optical and electrical properties because of the different ordering of the molecules inside them in comparison to bulk materials. C. R. Martin, Chem. Mater, 1996, 8, 1739; J.-K. Lee, W.-K. Koh, W.-S. Chae, and Y.-R. Kim, Chem. Comm., 2002, 138. Microcylinders can serve as a medium for longitudinal ordering of smaller rod-like objects, such as carbon nanotubes, for enzyme immobilization, or for the preparation of composite nanostructures. Y. Dror, W. Salalha, R. L. Khalfin, Y. Cohen, A. L. Yarin, E. Zussman, Langmuir 2003, 19, 7012; J. C. Hulteen, C. R. Martin, J. Mater. Chem. 1997, 7, 1075. Polymer rods of varying aspect ratios can assemble into various structures that could find applications as colloidal liquid crystals, pH-, electrolyte- and biologically-sensitive gels, photonic crystals of non-trivial symmetry, etc. For decades, the entropic self-assembly of rod-like colloidal particles has been of intense interest, but it has been studied mainly with viruses, and in a few cases with inorganic particles, as suitable anisotropic particles were not readily available. P. A. Forsyth, Jr. S. Mar{hacek over (c)}elja, D. J. Mitchell, B. W. Ninham, Adv. Colloid Interface Sci. 1978, 9, 37; G. J. Vroege, H. N. W. Lekkerkerker, Rep. Prog. Phys. 1992, 55, 1241; Z. Dogic, S. Fraden, Phys. Rev. Lett. 1997, 78, 2417; M. Adams, Z. Dogic, S. L. Keller, S. Fraden, Nature 1998, 393, 349; M. P. B. van Bruggen, F. M. van der Kooij, H. N. W. Lekkerkerker, J. Phys.: Condens. Matter 1996, 8, 9451; G. A. Vliegenthart, A. van Blaaderen, H. N. W. Lekkerkerker, Faraday Discuss. 1999, 112, 173; F. M. van der Kooij, H. N. W. Lekkerkerker, Phys. Rev. Lett. 2000, 84, 781.
Polymer structures in the form of nano-sized fibers, tubes, and “pencils” have been prepared using a “template synthesis,” which entails synthesizing the desired material within the cylindrical pores of a membrane (either inorganic or organic) or other porous structures, such as zeolites or mesoporous silica. V. M. Cepak, C. R. Martin, Chem. Mater. 1999, 11, 1363; M. Steinhart, J. H. Wendorff, A. Greiner, R. B. Wehrspohn, K. Nielsch, J. Schilling, J. Choi, U. Gösele, Science 2002, 296, 1997; S. Ai, G. Lu, Q. He, J. Li, J. Am. Chem. Soc. 2003, 125, 11140; S. I. Moon, T. J. McCarthy, Macromolecules 2003, 36, 4253; C.-Y. Peng, W. J. Nam, S. J. Fonash, B. Gu, A. Sen, K. Strawhecker, S. Natarajan, H. C. Foley, S. H. Kim, J. Am. Chem. Soc. 2003, 125, 9298; H. L. Frisch, J. E. Mark, Chem. Mater. 1996, 8, 1735; M. Fu, Y. Zhu, R. Tan, G. Shi, Adv. Mater. 2001, 13, 1874. Depending on the material and its interactions with the pore walls, the polymer nanocylinders formed may be solid or hollow tubes. Their synthesis can be achieved via polymerization of the corresponding monomer inside the pores or by infiltration of melted polymer or polymer solution trough the porous medium. The length of the polymer fibers formed is equal to the membrane thickness and their diameter is close to the average diameter of the template pores. Although this method provides good control over the particle sizes, it has a few major disadvantages: (i) in order to synthesize particles with desired dimensions, an appropriate template has to be found or prepared; and (ii) after completing the particle synthesis, additional treatment procedures are necessary to remove the template. In most cases, these procedures are expensive and not environmentally friendly since they include using a concentrated solution of sodium hydroxide (in the case inorganic membranes) or organic solvents (for organic templates). In general, the synthesis of large amounts of polymer cylinders using a template method is limited and costly.
An alternative technique for production of polymer nanofibers is the electro-spinning method. Y. Dror, W. Salalha, R. L. Khalfin, Y. Cohen, A. L. Yarin, and E. Zussman, Langmuir, 2003, 19, 7012; A. Theron, E. Zussman, and A. L. Yarin, Nanotechnology, 2001, 12, 384. In this method, an electrostatic field is created between a pending drop of a polymer solution and a rotating disk. The electrostatic forces draw a jet of the polymer solution, which solidifies upon solvent evaporation, and the resulting nanofibers, with lengths in the range of hundreds of micrometers, are deposited on the grounded disk. Besides the special equipment needed, the polymer solution used has to be amenable to electro-spinning. Moreover this method leads to a production of long fibers and does not provide a means for controlling fiber length. Similarly to the template method, very limited amounts of rods can be obtained and scalability is not easy.
Polymer nanocylinders can also be formed by self-assembly of block copolymer molecules. J. Ding, G. Liu, M. Yang, Polymer 1997, 38, 5497; Y. Yu, A. Eisenberg, J. Am. Chem. Soc. 1997, 119, 8383; G. Liu, X. Yan, S. Duncan, Macromolecules 2002, 35, 9788; J. Raez, J. P. Tomba, I. Manners, M. A. Winnik, J. Am. Chem. Soc. 2003, 125, 9546. Emulsion polymerization of tetrafluoroethylene in the presence of rod-like surfactant micelles has also yielded nano-sized cylindrical polymer particles. C. U. Kim, J. M. Lee, S. K. Ihm, J. Fluorine Chem. 1999, 96, 11.
Rod-like cylindrical particles on the micrometer scale could also form the basis of materials with unique and advantageous properties, yet very few processes for making such particles have been developed. Due to the lack of methods for facile fabrication of microrods, virtually every material assembled from micron-sized particles has been formed from spheres of silica or polymer latex. Y. Xia, B. Gates, Y. Yin, Y. Lu, Adv. Mater. 2000, 12, 693. Thus, there remains a need in the art for a method for forming polymer microrods in a cost-effective and scalable manner.